SPECIAL SECTION ON ENERGY EFFICIENT WIRELESS COMMUNICATIONS WITH ENERGY HARVESTING AND WIRELESS POWER TRANSFER Received October 16, 2017, accepted October 24, 2017, date of publication November 1, 2017, date of current version November 28, 2017. Digital Object Identifier 10.1109/ACCESS.2017.2768508 Securing Untrusted RF-EH Relay Networks Using Cooperative Jamming Signals AHMED EL SHAFIE 1 , (Member, IEEE), ASMA MABROUK 2 , KAMEL TOURKI 3 , (Senior Member, IEEE), NAOFAL AL-DHAHIR 1 , (Fellow, IEEE), AND RIDHA HAMILA 4 , (Senior Member, IEEE) 1 Electrical Engineering Department, The University of Texas at Dallas, Richardson, TX 75080, USA 2 HANA Research Laboratory, National School of Computer Sciences, Université de la Manouba, Manouba 2010, Tunisia 3 Mathematical and Algorithmic Sciences Laboratory, France Research Center, Huawei Technologies France SASU, 92100 Huawei, France 4 Department of Electrical Engineering, Qatar University, Doha, Qatar Corresponding author: Ahmed El Shafie (ahmed.salahelshafie@gmail.com) This work was supported by NPRP from the Qatar National Research Fund (a member of Qatar Foundation) under Grant 8-627-2-260. The statements made herein are solely the responsibility of the authors. ABSTRACT We propose a new scheme to secure a wireless-powered untrusted cooperative-communication network, where a legitimate source node (Alice) transmits her information messages to a legitimate destination node (Bob) through the multiple amplify-and-forward untrusted relays. The relay nodes are assumed to be honest but curious nodes; hence, they are trusted at the service level but are untrusted at the information level. To reduce the energy consumption of the network, only one relay node is selected in each time slot to forward Alice’s information signal. We assume a power-splitting-based energy-harvesting scheme, where each relay node splits its received signal into information and energy streams. Since the relay nodes are assumed to be untrusted at the information level, they attempt to decode the information intended to Bob while harvesting energy at the same time. When the relaying mode is selected, the scheme is realized over two non-overlapping time phases. To prevent any information leakage to the untrusted relay nodes, Bob and a cooperative jammer (John) inject jamming (artificial noise) signals during the first phase. During the second phase, the untrusted relay nodes that will not be forwarding the information signal must harvest energy to accumulate more energy to help Alice in future time slots. Moreover, the cooperative jammer will jam the untrusted relays to further power their batteries and prevent them from decoding the information-forwarding relay signal in case they decided to cheat and decode it. We model the battery state transitions at each relay as a finite-state Markov chain and analyze it. Our numerical results show the security gains of our proposed scheme relative to two benchmark schemes. INDEX TERMS Battery, energy harvesting, secrecy, untrusted relaying, relay selection. I. INTRODUCTION Radio frequency (RF) energy-harvesting (EH) schemes have gained increased interest recently as a promising solution to energize the battery-based wireless communication nodes in a wireless communication network [1]. Under the RF-EH paradigm, the wireless nodes convert the ambient RF transmissions into a direct current (DC) electricity. This prolongs their battery lifetimes with self-sufficient energy supply. Since RF transmissions can simultaneously carry both information and energy, an EH node is capable of decoding the information in a signal and converting a portion of that signal into energy. This motivates simultaneous wireless information and power transfer (SWIPT) [2]. The efficiency of RF-EH schemes is a function of many parameters including the network topology as mentioned in, e.g., [3] and the references therein. In particular, due to channel randomness, where the channel gain can be low for some times and high for other times, and its monotonically-decreasing behavior with distance, only wireless nodes that are relatively close to the transmitting nodes (e.g., base-stations or uplink users) can benefit from the energy carried over the RF transmissions. These wireless nodes do not only collect more wireless energy during the downlink (DL) phase but also use less transmit power during the uplink (UL) phase as well. In contrast, the nodes that are distant from the base-station will harvest less energy and will have to use more transmit powers during the uplink phases. Hence, it is necessary to have cooperative relay nodes in the middle between the base-station and the legitimate users to mitigate VOLUME 5, 2017 2169-3536  2017 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. 24353